An ink-jet printer includes a pen in which small droplets of ink are formed and ejected toward a printing medium. Such pens include printheads with orifice plates with several very small nozzles through which the ink droplets are ejected. Adjacent to the nozzles are ink chambers, where ink is stored prior to ejection through the nozzle. Ink is delivered to the ink chambers through ink channels that are in fluid communication with an ink supply. The ink supply may be, for example, contained in a reservoir part of the pen.
Ejection of an ink droplet through a nozzle may be accomplished by quickly heating a volume of ink within the adjacent ink chamber. The thermal process causes ink within the chamber to superheat and form a vapor bubble which, in turn, forces a drop of ink through the nozzle. This process is called "firing." The ink in the chamber may be heated with, for example, a resistor that is aligned adjacent to the nozzle.
Ejection of an ink droplet through a nozzle may also be accomplished through use of a piezoelectric element. A piezoelectric element is responsive to a control signal for abruptly compressing a volume of ink in the firing chamber thereby producing a pressure wave that forces the ink drops through the printhead orifices.
As the ink-jet pen traverses the surface of the print medium, droplets of ink are fired from the ink chambers, through the nozzles and toward the print medium. The droplets strike the medium and then dry to form dots that, when viewed together, create the printed image.
Good print quality is an important consideration in ink-jet printing. In order to print sharper and more definite images, either the printer resolution or the printer gray scale capabilities, or both, must be increased. Printer resolution is generally referred to as the number of dots per inch (dpi) the printer is capable of producing.
Gray scale print capability is the ability to produce printed images possessing shades of color which are only slightly distinguishable from one another. Gray scale print capability is often referred to or measured in terms of character darkness (i.e., blackness). Character darkness is also referred to as optical density.
In ink-jet printing, optical density has been varied in a step-wise manner only. One approach to increasing the optical density is to put multiple drops having the same optical density in the same position on the print medium. However, such an approach generally requires multiple passes of the pen thus, taking longer to print and requiring a significant amount of ink. Additionally, it is difficult to align multiple droplets in the same position. Poorly aligned droplets can result in printed images that appear fuzzy and soft. Moreover, multiple ink droplets prolong drytime, slowing print speed.
Another approach to varying optical density is to provide different ink reservoirs, in a multi-reservoir pen, with differing colorant loads of the same color ink. Alternatively, separate ink-jet pens with differing dye loads of the same color ink may be used. Dye load is a measure of the concentration of dye typically dissolved in an ink vehicle, such as, a mixture of water and organic solvent. With each of these approaches, ink from a plurality of ink supplies, typically three ink supplies, one of each light, medium and dark optical density ink, are combined on a print medium in order to improve gray scale capability. Thus, for a color ink-jet print system, which generally requires subtractive primary ink colors cyan, magenta and yellow, nine separate ink supplies (not including black ink) would be required. Accordingly, these approaches require a number of ink reservoirs or pens and, hence are expensive, complex systems that only vary optical density in a step-wise manner with a limited number of gray scale levels.
The present invention provides a variable optical density print system and method for continuous variation of the optical density among successive droplets expelled from the same nozzle. Having the ability to continuously vary the optical density of successive droplets provides the best possible gray scale control with a virtually limitless number of gray scale levels achieved. When combined with an ink-jet pen having sufficient resolution capability, the present invention generally provides printed images having excellent sharpness and definition.
The variable optical density print system of the present invention generally comprises two fluid channels, one channel in fluid communication with a basis fluid supply and the other channel in communication with a colorant concentrate fluid supply. Both fluid channels are in fluid communication with a single ink firing chamber. Depending upon the desired optical density of a given droplet, appropriate amounts of basis fluid and colorant concentrate fluid are delivered to the firing chamber by briefly opening a microvalve positioned within each fluid channel. The fluids mix in the firing chamber and are then ejected as a single ink droplet which possesses the desired optical density.